Method for Producing Heterogeneous Catalysts Containing Metal Nanoparticles

ABSTRACT

A monomer is added to a solvent containing metal salt and porous support materials and the solvent is stirred for a period of time to distribute and fix the metal in the pores of the support materials. The solids that are dispersed in the solvent are then separated from the liquid, dried and calcined to form heterogeneous catalysts. The monomer that is added is of a type that can be polymerized in the solvent to form oligomers or polymers, or both. When forming heterogeneous catalysts containing platinum, acrylic acid is selected as the preferred monomer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/866,566, filed Nov. 20, 2006, and U.S.Provisional Patent Application Ser. No. 60/867,335, filed Nov. 27, 2006,both of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the production of supportedcatalysts, and more particularly, to a method for making a heterogeneouscatalyst containing metal nanoparticles dispersed throughout the supportmaterial for the heterogeneous catalyst.

2. Description of the Related Art

Many industrial products such as fuels, lubricants, polymers, fibers,drugs, and other chemicals would not be manufacturable without the useof catalysts. Catalysts are also essential for the reduction ofpollutants, particularly air pollutants created during the production ofenergy and by automobiles. Many industrial catalysts are composed of ahigh surface area support material upon which chemically active metalnanoparticles (i.e., nanometer sized metal particles) are dispersed. Thesupport materials are generally inert, ceramic type materials havingsurface areas on the order of hundreds of square meters/gram. This highspecific surface area usually requires a complex internal pore system.The metal nanoparticles are deposited on the support and dispersedthroughout this internal pore system, and are generally between 1 and100 nanometers in size.

Processes for making supported catalysts go back many years. One suchprocess for making platinum catalysts, for example, involves thecontacting of a support material such as alumina with a metal saltsolution such as chloroplatinic acid. The salt solution “impregnates” orfills the pores of the support during this process. Following theimpregnation, the support containing the salt solution would be airdried, causing the metal salt to precipitate within the pores. Thesupport containing the crystallized metal salt would then be exposed toa hydrogen or carbon monoxide gas environment, reducing the solid metalsalt to metal particles.

Another process for making supported catalysts involves the steps ofcontacting a support material with a metal salt solution and reducingthe metal ions to metal particles in situ using suitable reducingagents. The following are examples of this process. U.K. Patent No.1,282,138 teaches methods for preparing metal catalysts using differentreducing agents, including glucose, hydroxylamine hydrochloride, andhydrazine. U.S. Pat. No. 4,086,275 teaches methods for preparing coppercatalysts using sodium borohydride as the in situ reducing agent. U.S.Pat. No. 4,835,131 teaches methods for preparing molybdenum on silicacatalyst, copper on gamma-alumina catalyst, silver on silica catalystand silver on gamma-alumina catalyst. The reducing agents used toprepare these catalysts include hydrazine, ammonium hydroxide, andformaldehyde. U.S. Pat. Nos. 5,275,998 and 5,275,999 teach methods forpreparing metal catalysts on carbon support and on alumina support usingdifferent reducing agents, including hydrazine hydrate, ascorbic acid,and sodium borohydride. According to these patents, supported catalystshaving very small metal particle size (average size not greater than 2nanometers) can be produced if the preparation steps are carried out inthe presence of ethylene and/or acetylene (U.S. Pat. No. 5,275,998) orin the presence of carbon monoxide (U.S. Pat. No. 5,275,998). U.S. Pat.No. 6,686,308 teaches methods for preparing metal catalysts on silicausing sodium citrate or potassium citrate as the reducing agent. Thispatent also teaches the use of colloid stabilizers including sodiumsulfanilate, and discloses that it is preferable to use colloidstabilizers that can also act as reducing agents, namely ammoniumcitrate, potassium citrate, and sodium citrate.

SUMMARY OF THE INVENTION

The present invention provides additional methods for preparingsupported metal catalysts. According to an embodiment of the presentinvention, a monomer is added to a solvent containing metal salt andporous support materials and the solvent is stirred for a period of timeto precipitate and/or reduce the metal salt in the pores of the supportmaterials. The solids that are dispersed in the solvent are thenseparated from the liquid, dried and calcined to form the supportedcatalyst. The monomer that is added to the solvent is of a type that canbe polymerized in the solvent to form oligomers or polymers, or both.Acrylic acid may be used as such a monomer.

According to another embodiment of the present invention, a supportedcatalyst is prepared by first forming an interim supported metalcatalyst through reduction or precipitation, and then carrying out thefurther steps of mixing the interim supported metal catalyst and metalsalt of the same metal type in a solvent, adding a fixing agent to thesolvent and stirring the solvent to precipitate or reduce the metal saltin the pores of the support materials for the catalyst, separating outthe solid in the solvent, drying the separated solid, and calcining it.The fixing agent for causing the reduction or precipitation during theprocess for forming the interim supported metal catalyst is selectedbased on the particular metal-support combination being used. Likewise,the fixing agent that is added to the solvent is selected based on theparticular metal-support combination being used.

According to still another embodiment of the present invention, asupported catalyst is prepared by first forming an interim supportedmetal catalyst through reduction or precipitation, and then carrying outthe further steps of mixing the interim supported metal catalyst andmetal salt of a different metal type in a solvent, adding a fixing agentto the solvent, stirring the solvent, separating out the solid in thesolvent, drying the separated solid, and calcining it. At least one ofthe fixing agents for causing the reduction or precipitation during theprocess for forming the interim supported metal catalyst and the fixingagent that is added to the solvent is a monomer of a type that can bepolymerized in the solvent to form oligomers or polymers, or both.Acrylic acid may be used as such a monomer.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a process flow block diagram of a method for making catalystsaccording to a first embodiment of the present invention.

FIG. 2 is a process flow block diagram of a method for making catalystsaccording to a second embodiment of the present invention.

FIG. 3 is a process flow block diagram of a method for making catalystsaccording to a third embodiment of the present invention.

FIGS. 4A-4D are schematic representations of different engine exhaustsystems in which catalysts made according to embodiments of the presentinvention may be used.

FIG. 5 is an illustration of a catalytic converter with a cut-awaysection that shows a substrate onto which catalysts made according toembodiments of the present invention are coated.

FIG. 6 is a flow diagram illustrating the steps for preparing an engineexhaust catalyst.

DETAILED DESCRIPTION

FIG. 1 shows a process flow block diagram of a method for makingcatalysts according to a first embodiment of the present invention. Instep 10, a monomer is added to a solvent containing metal salt andsupport materials. The monomer that is added to the solvent is of a typethat has some capability of interacting with the metal in the solventand can be polymerized in the solvent to form oligomers or polymers, orboth. Formation of oligomers and/or polymers in situ (i.e., in the freesolvent and/or in the pores of the support material) is desirablebecause they help stabilize the growth of nanoparticles. Acrylic acidmay be used as such a monomer, and is the preferred monomer forpreparing platinum catalysts on alumina supports. Other types ofmonomers that could be used, depending on a particular metal-supportcombination, include vinyl pyrrolidone, vinyl acetate, acrylamide,acrylic anhydride, sodium acrylate, glycidyl methacrylate, methacrylicacid, methacrylic anhydride, methyl methacrylate, 2-aminoethylmethacrylate hydrochloride, 1-vinylimidazole, allylamine, diallylamine,4-vinyl benzoic acid, 3-aminopropylmethyldiethoxysilane, 2-hydroxyethylacrylate, 4-acetoxy styrene, and mixtures thereof.

In addition to the monomer, a fixing agent, which may be a reducingagent, a precipitating agent or a hybrid reducing-precipitating agent,may be added in step 10 to the solvent containing metal salt and supportmaterials. Suitable fixing agents include: ascorbic acid, fumaric acid,acetic acid, maleic acid, H₂, CO, N₂H₄, NH₂OH, alcohols, citrates suchas sodium, potassium and ammonium citrate, alkali metal borohydridessuch as sodium and potassium borohydride, glycols, and mixtures thereof.

The solvent may be any liquid within which metal salt is suitablysoluble, and which is sufficiently pure enough and can be removed fromthe support materials by evaporation, filtering, pump evacuation,centrifuge, or other similar means. Such solvents include, but are notlimited to, water, alcohol, and other organic solvents. Preferably,water or double de-ionized water is used. Alcohols that are suitableinclude, but are not limited to, methanol and ethanol and theirmixtures, with and without water. Other organic solvents includetetrahydrofuran, acetic acid, ethylene glycol, N-methylpyrrolidone,dimethylformamide, dimethylacetalmide, and their mixtures, with andwithout water.

The metal salt may include one or more of the following metals: Pt, Pd,Ru, Rh, Re, Ir, Os, Fe, Co, Ni, Cu, Ag, Au, Zn, Cd, In, Ga, Sn, Pb, Bi,Sb, Ti, Zr, Cr, Mo, W, V, Nb and Mn. Of the foregoing, soluble salts ofPt, Pd, Ru. Rh, Re, Cu, Au, Re, Ir, Os and Ag are preferable. Pt saltsthat are suitable include Pt(NO₃)₂, (NH₃)₄Pt(NO₃)₂, H₂PtCl₆, K₂PtCl₄,(NH₃)₄Pt(OH)₂, and Cl₄Pt(NH₃)₂. Ag and Cu salts that are suitableinclude AgNO₃, AgCH₃COO, Cu(NO₃)₂, Cu(CH₃COO)₂, andCu(ll)acetylacetonate. Pd salts that are suitable include Pd(NH₃)₄(NO₃)₂and Pd(NO₃)₂. The concentration of the metal salt in the resultingsolution is preferably between 10⁻⁴ M and 0.1 M. The concentration ofthe metal salt in the resulting solution will depend on the targetweight loading of the final catalyst.

The support materials may be alumina, silica, oxides of vanadium, oxidesof titanium, oxides of zirconium, oxides of iron, cerium oxides, carbon,zeolites, molecular sieves, and various combinations thereof. Any ofthese support materials may be doped with lanthanum, other rare earthelements, alkali metals, alkaline earth metals, sulfur, selenium,tellurium, phosphorus, arsenic, antimony, or bismuth. The doping of thesupport materials may be carried out prior to, during, or even after theprocesses shown in FIGS. 1-3.

The solvent containing the metal salt and the support materials may beprepared by first adding the support materials in powder form into thesolvent and mixing the solvent. Sufficient agitation to keep the supportmaterials in suspension within the solution is desirable. If necessary,the temperature may be adjusted during this step. Typically, ambienttemperature or room temperature is used, within the range of 15 to 30°C. Metal salt is then added to the solvent in either dissolved form aspart of a salt solution or solid form. After the metal salt is added ineither dissolved form as part of a salt solution or solid form, thesolvent is mixed. Sufficient agitation to keep the support materials insuspension is desirable. Agitation is also required to fully dissolvethe metal salt within the solution and reduce any salt concentrationgradients within the solution. Typically, ambient temperature or roomtemperature is used, within the range of 15 to 30° C. The pH andtemperature of the solution may, however, be adjusted at this point, ifdesired. If the temperature or pH of the solution is adjusted,additional mixing is carried out.

Alternatively, the solvent containing the metal salt and the supportmaterials may be prepared by first adding the metal salt in eitherdissolved form as part of a salt solution or solid form into the solventand mixing the solvent for a time period, and then adding the supportmaterials into the solvent. As another alternative, the metal salt andthe support materials may be added to the solvent concurrently and thenmixed together in the solvent.

After the monomer is added in step 10, the solvent is stirred (step 12).Sufficient agitation to keep the support materials in suspension isdesirable. Mixing is carried out for a time period that is long enoughto cause the precipitation and/or reduction of the metal salt in thepores of the support materials. During this step, based on the type ofmonomer that is added in step 10 and the conditions under which step 12is carried out, the mixture may be heated or subjected to ultravioletlight, or polymerization initiators, such as AlBN or various types ofperoxides, may be added to the mixture, so as to initiate or increasethe polymerization of the monomer that is added in step 10.

In step 14, the support materials are separated from the solvent by anyconvenient method, such as evaporation, filtration, pump evacuation, orcentrifuge. Then, in step 16, the separated support materials are driedat an elevated temperature between 100° C. and 150° C., preferably about120° C. In step 18, the separated support materials are ground into finepowder and calcined in air at a temperature of 500° C. or higher. Thecalcination is typically carried out for 2 to 8 hours at the elevatedtemperature, and removes any organic residues such as any organicpolymer that was formed in situ during step 12 and remaining in thepores of the separated support materials. The separated supportmaterials that have been subjected to grinding and calcination in step18 represent the finished supported catalyst.

FIG. 2 shows a process flow block diagram of a method for makingcatalysts according to a second embodiment of the present invention. Instep 20, a fixing agent, which may be a reducing agent, a precipitatingagent or a hybrid reducing-precipitating agent, is added to a solventcontaining metal salt and support materials. The solvent, metal salt,and support materials may be of any type described above in connectionwith the first embodiment. Also, the solvent containing metal salt andsupport materials is prepared in the manner described above for thefirst embodiment. Suitable fixing agents include a monomer, such asacrylic acid, vinyl pyrrolidone, vinyl acetate, acrylamide, acrylicanhydride, sodium acrylate, glycidyl methacrylate, methacrylic acid,methacrylic anhydride, methyl methacrylate, 2-aminoethyl methacrylatehydrochloride, 1-vinylimidazole, allylamine, diallylamine, 4-vinylbenzoic acid, 3-aminopropylmethyldiethoxysilane, 2-hydroxyethylacrylate, 4-acetoxy styrene, and mixtures thereof, and others, such asascorbic acid, fumaric acid, acetic acid, maleic acid, H₂, CO, N₂H₄,NH₂OH, alcohols, citrates such as sodium, potassium and ammoniumcitrate, alkali metal borohydrides such as sodium and potassiumborohydride, glycols, and mixtures thereof.

After the fixing agent is added in step 20, the solvent is stirred (step22). Sufficient agitation to keep the support materials in suspension isdesirable. Mixing is carried out for a time period long enough tocomplete the precipitation and/or reduction of the metal salt in thepores of the support materials. In step 24, the support materials areseparated from the solvent by any convenient method, such asevaporation, filtration, pump evacuation, or centrifuge. Then, in step26, the separated support materials are dried at an elevated temperaturebetween 100° C. and 150° C., preferably about 120° C. In step 28, theseparated support materials are ground into fine powder and calcined inair at a temperature of 500° C. or higher. The calcination is typicallycarried out for 2 to 8 hours at the elevated temperature. The separatedsupport materials that have been subjected to grinding and calcinationin step 28 represent the interim supported catalyst.

After step 28, a portion of the separated support materials is mixedwith a metal salt having the same metal type as the metal salt added instep 20 in a solvent (step 30). The solvent may be of any type describedabove in connection with the first embodiment. As described in the firstembodiment, the support materials may be added first into the solvent,or the metal salt may be added first into the solvent, or the two may beadded at about the same time into the solvent. Regardless of the orderby which the support materials and the metal salt are added into thesolvent, prior to step 32, the solvent is mixed sufficiently to keep thesupport materials in suspension within the solution and to fullydissolve the metal salt within the solution and reduce any saltconcentration gradients within the solution. Typically, ambienttemperature or room temperature is used, within the range of 15 to 30°C. The pH and temperature of the solution may, however, be adjusted atthis point, if desired. If the temperature or pH of the solution isadjusted, additional mixing is carried out.

In step 32, a fixing agent, which may be a reducing agent, aprecipitating agent or a hybrid reducing-precipitating agent, is addedto the solvent. Suitable fixing agents include a monomer, such asacrylic acid, vinyl pyrrolidone, vinyl acetate, acrylamide, acrylicanhydride, sodium acrylate, glycidyl methacrylate, methacrylic acid,methacrylic anhydride, methyl methacrylate, 2-aminoethyl methacrylatehydrochloride, 1-vinylimidazole, allylamine, diallylamine, 4-vinylbenzoic acid, 3-aminopropylmethyldiethoxysilane, 2-hydroxyethylacrylate, 4-acetoxy styrene, and mixtures thereof, and others, such asascorbic acid, fumaric acid, acetic acid, maleic acid, H₂, CO, N₂H₄,NH₂OH, alcohols, citrates such as sodium, potassium and ammoniumcitrate, alkali metal borohydrides such as sodium and potassiumborohydride, glycols, and mixtures thereof.

After the fixing agent is added in step 32, the solvent is stirred (step34). Sufficient agitation to keep the support materials in suspension isdesirable. Mixing is carried out for a time period that is long enoughto complete the precipitation and/or reduction of the metal salt in thepores of the support materials. In step 36, the support materials areseparated from the solvent by any convenient method, such asevaporation, filtration, pump evacuation, or centrifuge. Then, in step38, the separated support materials are dried at an elevated temperaturebetween 100° C. and 150° C., preferably about 120°0 C. In step 40, theseparated support materials are ground into fine powder and calcined inair at a temperature of 500° C. or higher. The calcination is typicallycarried out for 2 to 8 hours at the elevated temperature. The separatedsupport materials that have been subjected to grinding and calcinationin step 40 represent the final supported catalyst.

In the second embodiment, steps 26 and 28 may be omitted. In such acase, the support materials that are separated from the solvent in step24 represent the interim supported catalyst and a portion thereof isredispersed in a solvent and mixed with metal salt in step 30.

FIG. 3 shows a process flow block diagram of a method for makingcatalysts according to a third embodiment of the present invention.Steps 50, 52, 54, 56, 58, 60, 62, 64, 66, 68 and 70 correspondrespectively to steps 20, 22, 24, 26, 28, 30, 32, 34, 36, 38 and 40 ofthe second embodiment and are carried out in the same manner, except forstep 60. In step 60, metal salt having a metal type that is not the sameas the metal salt in step 50 is mixed with the interim supportedcatalyst in a solvent. Also, in the third embodiment, at least one ofthe fixing agent added in step 50 and the fixing agent added in step 62is a monomer of a type that has some capability of interacting with themetal in the solvent and can be polymerized in the solvent to formoligomers or polymers, or both. Acrylic acid may be used as such amonomer, and is the preferred monomer for preparing platinum catalystson alumina supports. Other types of monomers that could be used,depending on a particular metal-support combination, include vinylpyrrolidone, vinyl acetate, acrylamide, acrylic anhydride, sodiumacrylate, glycidyl methacrylate, methacrylic acid, methacrylicanhydride, methyl methacrylate, 2-aminoethyl methacrylate hydrochloride,1-vinylimidazole, allylamine, diallylamine, 4-vinyl benzoic acid,3-aminopropylmethyldiethoxysilane, 2-hydroxyethyl acrylate, 4-acetoxystyrene, and mixtures thereof.

As with the second embodiment, in the third embodiment, steps 56 and 58may be omitted. In such a case, the support materials that have beenseparated from the solvent in step 54 represent the interim supportedcatalyst and a portion thereof is redispersed in a solvent and mixedwith metal salt in step 60.

Potential advantages of the method according to the second and thirdembodiments include: (1) the ability to synthesize catalysts with highermetal concentrations while maintaining high dispersions of metalparticles; and (2) allowing use of different (potentially incompatible)metal salts and/or reducing agents to provide enhanced control overparticle sizes and structure tuning.

FIGS. 4A-4D are schematic representations of different engine exhaustsystems in which supported metal catalysts according to embodiments ofthe present invention may be used. The combustion process that occurs inan engine 102 produces harmful pollutants, such as CO, varioushydrocarbons, particulate matter, and nitrogen oxides (NOx), in anexhaust stream that is discharged through a tail pipe 108 of the exhaustsystem.

In the exhaust system of FIG. 4A, the exhaust stream from an engine 102passes through a catalytic converter 104, before it is discharged intothe atmosphere (environment) through a tail pipe 108. The catalyticconverter 104 contains supported catalysts coated on a monolithicsubstrate that treat the exhaust stream from the engine 102. The exhauststream is treated by way of various catalytic reactions that occurwithin the catalytic converter 104. These reactions include theoxidation of CO to form CO₂, burning of hydrocarbons, and the conversionof NO to NO₂.

In the exhaust system of FIG. 4B, the exhaust stream from the engine 102passes through a catalytic converter 104 and a particulate filter 106,before it is discharged into the atmosphere through a tail pipe 108. Thecatalytic converter 104 operates in the same manner as in the exhaustsystem of FIG. 4A. The particulate filter 106 traps particulate matterthat is in the exhaust stream, e.g., soot, liquid hydrocarbons,generally particulates in liquid form. In an optional configuration, theparticulate filter 106 includes a supported catalyst coated thereon forthe oxidation of NO and/or to aid in combustion of particulate matter.

In the exhaust system of FIG. 4C, the exhaust stream from the engine 102passes through a catalytic converter 104, a pre-filter catalyst 105 anda particulate filter 106, before it is discharged into the atmospherethrough a tail pipe 108. The catalytic converter 104 operates in thesame manner as in the exhaust system of FIG. 4A. The pre-filter catalyst105 includes a monolithic substrate and supported catalysts coated onthe monolithic substrate for the oxidation of NO. The particulate filter106 traps particulate matter that is in the exhaust stream, e.g., soot,liquid hydrocarbons, generally particulates in liquid form.

In the exhaust system of FIG. 4D, the exhaust stream passes from theengine 102 through a catalytic converter 104, a particulate filter 106,a selective catalytic reduction (SCR) unit 107 and an ammonia slipcatalyst 110, before it is discharged into the atmosphere through a tailpipe 108. The catalytic converter 104 operates in the same manner as inthe exhaust system of FIG. 4A. The particulate filter 106 trapsparticulate matter that is in the exhaust stream, e.g., soot, liquidhydrocarbons, generally particulates in liquid form. In an optionalconfiguration, the particulate filter 106 includes a supported catalystcoated thereon for the oxidation of NO and/or to aid in combustion ofparticulate matter. The SCR unit 107 is provided to reduce the NOxspecies to N₂. The SCR unit 107 may be ammonia/urea based or hydrocarbonbased. The ammonia slip catalyst 110 is provided to reduce the amount ofammonia emissions through the tail pipe 108. An alternativeconfiguration places the SCR unit 107 in front of the particulate filter106.

Alternative configurations of the exhaust system includes the provisionof SCR unit 107 and the ammonia slip catalyst 110 in the exhaust systemof FIG. 4A or 1C, and the provision of just the SCR unit 107, withoutthe ammonia slip catalyst 110, in the exhaust system of FIGS. 4A, 4B or4C.

As particulates get trapped in the particulate filter within the exhaustsystem of FIG. 4B, 4C or 4D, it becomes less effective and regenerationof the particulate filter becomes necessary. The regeneration of theparticulate filter can be either passive or active. Passive regenerationoccurs automatically in the presence of NO₂. Thus, as the exhaust streamcontaining NO₂ passes through the particulate filter, passiveregeneration occurs. During regeneration, the particulates get oxidizedand NO₂ gets converted back to NO. In general, higher amounts of NO₂improve the regeneration performance, and thus this process is commonlyreferred to as NO₂ assisted oxidation. However, too much NO₂ is notdesirable because excess NO₂ is released into the atmosphere and NO₂ isconsidered to be a more harmful pollutant than NO. The NO₂ used forregeneration can be formed in the engine during combustion, from NOoxidation in the catalytic converter 104, from NO oxidation in thepre-filter catalyst 105, and/or from NO oxidation in a catalyzed versionof the particulate filter 106.

Active regeneration is carried out by heating up the particulate filter106 and oxidizing the particulates. At higher temperatures, NO₂assistance of the particulate oxidation becomes less important. Theheating of the particulate filter 106 may be carried out in various waysknown in the art. One way is to employ a fuel burner which heats theparticulate filter 106 to particulate combustion temperatures. Anotherway is to increase the temperature of the exhaust stream by modifyingthe engine output when the particulate filter load reaches apre-determined level.

The present invention provides embodiments of a supported metal catalystthat is to be used in the catalytic converter 104 shown in FIGS. 4A-4D,or generally as a catalyst in any vehicle emission control system,including as a diesel oxidation catalyst, a diesel filter catalyst, anammonia-slip catalyst, an SCR catalyst, or as a component of a three-waycatalyst.

FIG. 5 is an illustration of a catalytic converter with a cut-awaysection that shows a substrate 210 onto which engine exhaust catalystsaccording to embodiments of the present invention are coated. Theexploded view of the substrate 210 shows that the substrate 210 has ahoneycomb structure comprising a plurality of channels into which engineexhaust catalysts are flowed in slurry form so as to form coating 220 onthe substrate 210.

FIG. 6 is a flow diagram that illustrates the steps for preparing anengine exhaust catalyst using the substrate 210 of FIG. 5. In step 410,the supported metal catalyst, e.g., a platinum-palladium catalyst, isprepared. A monolithic substrate, such as substrate 210 shown in FIG. 5,is provided in step 414. Exemplary monolithic substrates include thosethat are ceramic (e.g., cordierite), metallic, or silicon carbide based.Then, the supported metal catalyst in powder form is mixed in a solventto form a slurry (step 416) and the slurry is then coated onto themonolithic substrate (step 418).

The following examples serve to explain and illustrate the embodimentsof the present invention.

Example 1: Pt (1%) on Alumina

Add 4 g of alumina (alumina having a BET surface area of 140 m²/g) to 17ml of H₂O and stir for 20 minutes at room temperature. Add 1.0 ml ofPt(NO₃)₂ (40 mg/ml Pt) solution into the system and stir at roomtemperature for 60 minutes. Add 0.5 ml acrylic acid (having 99% purity)into the system and continue to stir at room temperature for 2 hours.Filter to separate out the solid from the system. Dry the separatedsolid at 120° C. for 2 hours, grind it into fine powder, and calcine itin air for 2 hours at a temperature of 500° C.

Example 2: Pt (3%) on Alumina

Add 4 g of alumina (alumina having a BET surface area of 140 m²/g) to 17ml of H₂O and stir for 20 minutes at room temperature. Add 1.2 ml ofPt(NO₃)₂ (100 mg/ml Pt) solution into the system and stir at roomtemperature for 60 minutes. Add 1.5 ml acrylic acid (having 99% purity)into the system and continue to stir at room temperature for 2 hours.Filter to separate out the solid from the system. Dry the separatedsolid at 120° C. for 2 hours, grind it into fine powder, and calcine itin air for 2 hours at a temperature of 500° C.

Example 3: Pt (1%) on La-doped Alumina

Add 4 g of La-doped alumina (alumina doped with 4% of La₂O₃ by weighthaving BET surface area of 200 m²/g) to 19 ml of H₂O and stir for 20minutes at room temperature. Add 0.4 ml of Pt(NO₃)₂ (100 mg/ml Pt)solution into the system and stir at room temperature for 60 minutes.Add 0.5 ml acrylic acid (having 99% purity) into the system and continueto stir at room temperature for 2 hours. Filter to separate out thesolid from the system. Dry the separated solid at 120° C. for 2 hours,grind it into fine powder, and calcine it in air for 2 hours at atemperature of 500° C.

Example 4: Pt (3%) on La-doped Alumina

Add 4 g of La-doped alumina (alumina doped with 4% of La₂O₃ by weighthaving BET surface area of 200 m²/g) to 17.3 ml of H₂O and stir for 20minutes at room temperature. Add 1.2 ml of Pt(NO₃)₂ (100 mg/ml Pt)solution into the system and stir at room temperature for 60 minutes.Add 1.5 ml acrylic acid (having 99% purity) into the system and continueto stir at room temperature for 2 hours. Filter to separate out thesolid from the system. Dry the separated solid at 120° C. for 2 hours,grind it into fine powder, and calcine it in air for 2 hours at atemperature of 500° C.

Example 5: Pt (3%) on La-doped Alumina

Add 4 g of La-doped alumina (alumina doped with 4% of La₂O₃ by weighthaving BET surface area of 200 m²/g) to 17.3 ml of H₂O and stir for 20minutes at room temperature. Add 1.2 ml of Pt(NO₃)₂ (100 mg/ml Pt)solution into the system and stir at room temperature for 60 minutes.Add 1.5 ml acrylic acid (having 99% purity) into the system and continueto stir at room temperature for 10 minutes. Increase the temperature to100° C. and stir for 1 hour, and then, decrease the temperature to roomtemperature. Filter to separate out the solid from the system. Dry theseparated solid at 120° C. for 2 hours, grind it into fine powder, andcalcine it in air for 2 hours at a temperature of 500° C.

Example 6: Pt (3%) on La-doped Alumina

Add 4 g of La-doped alumina (alumina doped with 4% of La₂O₃ by weighthaving BET surface area of 200 m²/g) to 15.3 ml of H₂O and stir for 20minutes at room temperature. Add 1.2 ml of Pt(NO₃)₂ (100 mg/ml Pt)solution into the system and stir at room temperature for 60 minutes.Add 1.5 ml acrylic acid (having 99% purity) into the system and continueto stir at room temperature for 10 minutes. Then, add 18 mg of2,2-Azobisisobutyronitrile (AlBN) suspended in 2 ml of H₂O into thesystem and continue to stir at room temperature for 60 minutes. Filterto separate out the solid from the system. Dry the separated solid at120° C. for 2 hours, grind it into fine powder, and calcine it in airfor 2 hours at a temperature of 500° C.

Example 7: Pt (1%) on La-doped Alumina (Sequential Method)

Add 30 g of La-doped alumina (alumina doped with 4% of La₂O₃ by weighthaving BET surface area of 200 m²/g) to 145 ml of H₂O and stir for 20minutes at room temperature. Add 1.5 ml of Pt(NO₃)₂ (100 mg/ml Pt)solution into the system and stir at room temperature for 60 minutes.Add 3.75 ml acrylic acid (having 99% purity) into the system andcontinue to stir at room temperature for 2 hours. Filter to separate outthe solid from the system. Dry the separated solid at 120° C. for 2hours, grind it into fine powder, and calcine it in air for 2 hours at atemperature of 500° C. After calcining the separated solid, add 4 g ofit into 19.3 ml of H₂O and stir for 20 minutes at room temperature. Add0.2 ml of Pt(NO₃)₂ (100 mg/ml Pt) solution into the system and stir atroom temperature for 60 minutes. Add 0.5 ml acrylic acid (having 99%purity) into the system and continue to stir at room temperature for 2hours. Filter to separate out the solid from the system. Dry theseparated solid at 120° C. for 2 hours, grind it into fine powder, andcalcine it in air for 2 hours at a temperature of 500° C.

Example 8: Pt (3%) on La-doped Alumina (Sequential Method)

Add 10 g of La-doped alumina (alumina doped with 4% of La₂O₃ by weighthaving BET surface area of 200 m²/g) to 45 ml of H₂O and stir for 20minutes at room temperature. Add 1.5 ml of Pt(NO₃)₂ (100 mg/ml Pt)solution into the system and stir at room temperature for 60 minutes.Add 1.875 ml acrylic acid (having 99% purity) into the system andcontinue to stir at room temperature for 2 hours. Filter to separate outthe solid from the system. Dry the separated solid at 120° C. for 2hours, grind it into fine powder, and calcine it in air for 2 hours at atemperature of 500° C. After calcining the separated solid, add 4 g ofit into 19.3 ml of H₂O and stir for 20 minutes at room temperature. Add0.6 ml of Pt(NO₃)₂ (100 mg/ml Pt) solution into the system and stir atroom temperature for 60 minutes. Add 0.75 ml acrylic acid (having 99%purity) into the system and continue to stir at room temperature for 2hours. Filter to separate out the solid from the system. Dry theseparated solid at 120° C. for 2 hours, grind it into fine powder, andcalcine it in air for 2 hours at a temperature of 500° C.

Example 9: Pt+Pd (1%) on Alumina

Add 30 g of alumina (alumina having a BET surface area of 140 m²/g) into140 ml of H₂O and stir for 20 minutes at room temperature. Add 1.5 ml ofPt(NO₃)₂ (100.0 mg/ml Pt) into the system and continue to stir at roomtemperature for 60 min. Add 3.75 ml of acrylic acid (having 99% purity)and stir at room temperature for 2 hours. Filter to separate out thesolid from the system. Dry the separated solid at 130° C. for 90minutes, grind it into fine powder, and calcine it in air for 2 hours ata temperature of 500° C. After calcining the separated solid, add 2 g ofit into 9 ml of H₂O and stir for 20 minutes at room temperature. Add0.25 ml of Pd(NO₃)₂ (40 mg/ml Pd) and continue to stir at roomtemperature for 30 minutes. Add 0.662 g of ascorbic acid and stir atroom temperature for 1 hour. Filter to separate out the solid from thesystem. Dry the separated solid at 130° C. for 150 minutes, grind itinto fine powder, and calcine it in air for 1 hour at a temperature of500° C.

Example 10: Pd+Pt (1%) on Alumina

Add 30 g of alumina (alumina having a BET surface area of 140 m²/g) into140 ml of H₂O and stir for 20 minutes at room temperature. Add 3.75 mlof Pd(NO₃)₂ (40 mg/ml Pd) and continue to stir at room temperature for60 minutes. Add 9.93 g of ascorbic acid and stir at room temperature for1 hour. Filter to separate out the solid from the system. Dry theseparated solid at 130° C. for 150 minutes, grind it into fine powder,and calcine it in air for 1 hour at a temperature of 500° C. Aftercalcining the separated solid, add 4 g of it into 18 ml of H₂O and stirfor 20 minutes at room temperature. Add 0.2 ml of Pt(NO₃)₂ (100.0 mg/mlPt) into the system and continue to stir at room temperature for 60 min.Add 0.5 ml of acrylic acid (having 99% purity) and stir at roomtemperature for 1 hour. Filter to separate out the solid from thesystem. Dry the separated solid at 130° C. for 150 minutes, grind itinto fine powder, and calcine it in air for 1 hour at a temperature of500° C.

A commonly used metric for measuring catalytic efficiency of catalystsis the temperature at which 50% conversion of CO into CO₂ is observed.For simplicity, this temperature will be referred to herein as the T50temperature. Likewise, the T20 temperature corresponds to thetemperature at which 20% of CO will be oxidized into CO₂, and the T70temperature corresponds to the temperature at which 70% of CO will beoxidized into CO₂. Generally, higher conversion is observed at highertemperatures and lower conversion is observed at lower temperatures.

The T20, T50, T70 temperatures of catalysts differ depending on theconditions under which the conversion of CO into CO₂ is observed.Therefore, they are determined under conditions that simulate the actualoperating conditions of the catalyst as closely as possible. Thesupported catalysts produced in accordance with the embodiments of thepresent invention are useful as diesel exhaust catalysts, and thus theirT20, T50, T70 temperatures have been determined under simulated dieselexhaust conditions, which were as follows. A gas mixture having thecomposition: CO (1000 ppm), C₃H₈ (105 ppm), C₃H₆ (245 ppm), NO (450ppm), CO₂ (10%), O₂ (10%), and He (balance) is supplied into a fixed bedflow reactor containing 15 mg of catalyst powder mixed with 85 mg ofα-Al₂O₃ at a total flow rate of 300 ml/min. The reactor is heated fromroom temperature to 300° C. at 10° C./minute. As the reactor is heated,CO conversion (oxidation) was measured by use of mass spectrometry as afunction of temperature. Hydrocarbon conversion (oxidation) was alsomeasured as a function of temperature by use of mass spectrometry.

The tables below provide a comparison of the T20, T50, T70 temperaturesof the supported catalysts produced in accordance with the embodimentsof the present invention with the supported catalysts produced inaccordance with the prior art synthesis techniques, including diluteimpregnation with filtration (Prior Art I), in situ reduction usinghydrazine as the reducing agent (Prior Art II), and incipient wetness(Prior Art III). The T20, T50, T70 temperatures for each catalyst samplewere determined under simulated diesel exhaust conditions. Table 1 is acomparison of the T20, T50, T70 temperatures for a platinum catalysthaving a Pt weight loading of 1% supported on alumina. Table 2 is acomparison of the T20, T50, T70 temperatures for a platinum catalysthaving a Pt weight loading of 3% supported on alumina. Table 3 is acomparison of the T20, T50, T70 temperatures for a platinum catalysthaving a Pt weight loading of 1% supported on a lanthanum-doped alumina.Table 4 is a comparison of the T20, T50, T70 temperatures for a platinumcatalyst having a Pt weight loading of 3% supported on a lanthanum-dopedalumina.

TABLE 1 Synthesis T20 T50 T70 Method (° C.) (° C.) (° C.) Example 1 139151 163 Prior Art I 195 200 204 Prior Art II 177 202 212 Prior Art III188 195 199

TABLE 2 Synthesis T20 T50 T70 Method (° C.) (° C.) (° C.) Example 2 139151 161 Prior Art I 176 181 183 Prior Art II 160 166 170 Prior Art III183 188 190

TABLE 3 Synthesis T20 T50 T70 Method (° C.) (° C.) (° C.) Example 3 144156 173 Example 7 138 152 166 Prior Art I 204 210 215 Prior Art II 162212 218 Prior Art III 200 208 212

TABLE 4 Synthesis T20 T50 T70 Method (° C.) (° C.) (° C.) Example 4 123134 139 Example 5 116 129 134 Example 6 127 137 143 Example 8 122 134140 Prior Art I 182 188 191 Prior Art II 155 200 211 Prior Art III 180187 190

According to the first embodiment of the present invention, a monomer ofa type that has some capability of interacting with the metal in thesolvent and can be polymerized in the solvent to form oligomers orpolymers, or both, is added to the solvent containing the metal salt andsupport materials. Formation of oligomers and/or polymers in situ (i.e.,in the solvent) is desirable because they help stabilize the growth ofnanoparticles inside the pores of the support material. In Examples 1-4and 7-8, acrylic acid is used as such a monomer and is added to thesolvent containing the metal salt and support materials at roomtemperature, and the mixture is maintained at room temperature. Even atroom temperature, the in situ polymerization of the acrylic acid hasbeen observed. In Example 5, the temperature of the mixture was raisedto 100° C. to cause further polymerization of the acrylic acid in themixture. In Example 6, the temperature of the mixture was maintained atroom temperature, but AlBN was added to the mixture to cause furtherpolymerization of the acrylic acid in the mixture.

As a way to illustrate the advantages of in situ polymerization of amonomer, the T20, T50, T70 temperatures of the supported catalystsproduced in accordance with Examples 4-6 are compared with the supportedcatalysts produced in accordance with the prior art synthesis techniquesin which a pre-formed polymer is added to a solvent containing metalsalt and support materials as a fixing agent for the metal. Table 5 is acomparison of the T20, T50, T70 temperatures for a platinum catalysthaving a Pt weight loading of 3% supported on a lanthanum-doped alumina.In the Prior Art IV example, 2.0349 g of sodium polyacrylate (molecularweight=2100) was added to a solvent containing Pt(NO₃)₂ salt andlanthanum-doped alumina support materials as the pre-formed polymer andstirred at room temperature. In the Prior Art V example, 8.1394 g ofsodium polyacrylate (molecular weight=2100) was added to a solventcontaining Pt(NO₃)₂ salt and lanthanum-doped alumina support materialsas the pre-formed polymer and stirred at room temperature.

TABLE 5 Synthesis T20 T50 T70 Method (° C.) (° C.) (° C.) Example 4 123134 139 Example 5 116 129 134 Example 6 127 137 143 Prior Art IV 124 137143 Prior Art V 130 140 150

The advantages of in situ polymerization also are evident when theeffectiveness of the catalysts for hydrocarbon oxidation is considered.Table 6 is a comparison of the effectiveness for hydrocarbon oxidationof the supported catalysts produced in accordance with Examples 4-6against the effectiveness for hydrocarbon oxidation of the supportedcatalysts produced in accordance with the prior art synthesis techniquesin which a pre-formed polymer is added to a solvent containing metalsalt and support materials as a fixing agent for the metal. In Table 6,the T20_HC temperature corresponds to the temperature at which 20% ofC₃H₆ is oxidized; the T50_HC temperature corresponds to the temperatureat which 50% of C₃H₆ is oxidized; and the T70_HC temperature correspondsto the temperature at which 70% of C₃H₆ is oxidized.

TABLE 6 Synthesis T20_HC T50_HC T70_HC Method (° C.) (° C.) (° C.)Example 4 152 164 176 Example 5 153 163 166 Example 6 152 162 171 PriorArt IV 173 193 198 Prior Art V 201 208 210

Example 11—3.0% Pt, 1.5% Pd Supported Catalyst

To 10 L of de-ionized H₂O was added 1940 g of La-stabilized alumina(having a BET surface area of ˜200 m² g⁻¹) followed by stirring for 30minutes at room temperature. To this slurry was added 490.6 g ofPt(NO₃)₂ solution (12.23% Pt(NO₃)₂ by weight), followed by stirring atroom temperature for 60 minutes. Acrylic acid (750 mL, 99% purity) wasthen added into the system over 12 minutes and the resulting mixture wasallowed to continue stirring at room temperature for 2 hours. The solidLa-doped alumina supported Pt catalyst was separated from the liquid viafiltration, dried at 120° C. for 2 hours, ground into a fine powder, andcalcined in air for 2 hours at a temperature of 500° C. (heated at 8° C.min⁻¹) to give a 3% Pt material.

To 9.25 L of de-ionized H₂O was added 1822 g of the above 3% Pt materialfollowed by stirring for 20 minutes at room temperature. To this slurrywas added 194.4 g of Pd(NO₃)₂ solution (14.28% Pd(NO₃)₂ by weight),followed by stirring at room temperature for 60 minutes. An aqueousascorbic acid solution (930 g in 4.5 L of de-ionized H₂O) was then addedover 25 minutes followed by stirring for 60 minutes. The solid La-dopedalumina supported PtPd catalyst was separated from the liquid viafiltration, dried at 120° C. for 2 hours, ground into a fine powder, andcalcined in air for 2 hours at a temperature of 500° C. (heated at 8° C.min⁻¹) to give a 3% Pt, 1.5% Pd material.

Example 12—2.0% Pt, 1.0% Pd Supported Catalyst

To 10 L of de-ionized H₂O was added 2000 g of La-stabilized alumina(having a BET surface area of ˜200 m²g⁻¹) followed by stirring for 30minutes at room temperature. To this slurry was added 327.1 g ofPt(NO₃)₂ solution (12.23% Pt(NO₃)₂ by weight), followed by stirring atroom temperature for 60 minutes. Acrylic acid (500 mL, 99% purity) wasthen added into the system over 12 minutes and the resulting mixture wasallowed to continue stirring at room temperature for 2 hours. The solidLa-doped alumina supported Pt catalyst was separated from the liquid viafiltration, dried at 120° C. for 2 hours, ground into a fine powder, andcalcined in air for 2 hours at a temperature of 500° C. (heated at 8° C.min⁻¹) to give a 2% Pt material.

To 9.5 L of de-ionized H₂O was added 1900 g of the above 2% Pt materialfollowed by stirring for 20 minutes at room temperature. To this slurrywas added 135.3 g of Pd(NO₃)₂ solution (14.28% Pd(NO₃)₂ by weight),followed by stirring at room temperature for 60 minutes. An aqueousascorbic acid solution (647.2 g in 3.5 L of de-ionized H₂O) was thenadded over 25 minutes followed by stirring for 60 minutes. The solidLa-doped alumina supported PtPd catalyst was separated from the liquidvia filtration, dried at 120° C. for 2 hours, ground into a fine powder,and calcined in air for 2 hours at a temperature of 500° C. (heated at8° C. min⁻¹) to give a 2% Pt, 1% Pd material.

Example 13—Engine Exhaust Catalyst Having Supported Pt/Pd at 120 g/ft³

The supported PtPd catalyst powder (2.0% Pt, 1.0% Pd) prepared above(Example 12) was made into a washcoat slurry via addition to de-ionizedwater, milling to an appropriate particle size (typically with a d₅₀range from 3 to 7 μm), and pH adjustment to give an appropriateviscosity for washcoating. According to methods known in the art, thewashcoat slurry was coated onto a round cordierite monolith (Corning,400 cpsi, 5.66 inches×2.5 inches), dried at 120° C. and calcined at 500°C. to give the final coated monolith with a precious metal (Pt+Pd)loading of 120 g/ft³. The coated monolith was canned according tomethods known in the art and tested for CO emissions using a certifiedtesting facility on a light-duty diesel vehicle (model year 2005). TheCO emissions, as measured from the tail pipe of the light-duty dieselvehicle using bag data from the standard European MVEG test, wereobserved to be 0.222 g/km.

While particular embodiments according to the invention have beenillustrated and described above, those skilled in the art understandthat the invention can take a variety of forms and embodiments withinthe scope of the appended claims.

1. A method for producing a supported catalyst, comprising: adding amonomer to a solvent containing metal salt and support materials, themonomer being of a type that can be polymerized in the solvent to formoligomers or polymers, or both; stirring the solvent containing themetal salt, the support materials and the monomer; separating the liquidfrom the solid in the solvent; and calcining the separated solid toproduce the supported catalyst.
 2. The method according to claim 1,wherein the monomer comprises acrylic acid.
 3. The method according toclaim 2, wherein the metal salt comprises platinum salt and the supportmaterials are made of lanthanum-doped alumina.
 4. The method accordingto claim 1, further comprising the step of adding a fixing agent that isnot a monomer to the solvent containing metal salt and supportmaterials.
 5. The method according to claim 1, further comprising thestep of heating the solvent containing the metal salt, the supportmaterials and the monomer to increase the amount of polymerization ofthe monomer in the solvent.
 6. The method according to claim 1, furthercomprising the step of adding a polymerization initiator into thesolvent containing the metal salt, the support materials and the monomerto increase the amount of polymerization of the monomer in the solvent.7. A method for producing a supported catalyst, comprising: adding afixing agent to a first solvent containing metal salt of a predeterminedmetal type and support materials, stirring the first solvent, and thenseparating out the support materials from the first solvent; mixing aportion of the support materials separated out from the first solventand metal salt of the predetermined metal type into a second solvent;and adding a fixing agent to the second solvent, stirring the secondsolvent, and then separating out the support materials from the secondsolvent; and calcining the support materials separated out from thesecond solvent to produce the supported catalyst.
 8. The methodaccording to claim 7, wherein the fixing agent added to the firstsolvent and the fixing agent added to the second solvent are monomers ofa type that can be polymerized in the solvent to form oligomers orpolymers, or both.
 9. The method according to claim 8, wherein themonomer added to the first solvent and the monomer added to the secondsolvent comprise acrylic acid.
 10. The method according to claim 9,wherein the metal salt comprises platinum salt.
 11. The method accordingto claim 7, further comprising the step of calcining the supportmaterials separated out from the first solvent prior to the step ofmixing.
 12. The method according to claim 7, wherein the fixing agentcomprises a reducing agent.
 13. The method according to claim 7, whereinthe fixing agent comprises a precipitating agent.
 14. A method forproducing a supported catalyst, comprising: adding a fixing agent to afirst solvent containing metal salt of a first metal type and supportmaterials, stirring the first solvent, and then separating out thesupport materials from the first solvent; mixing a portion of thesupport materials separated out from the first solvent and metal salt ofa second metal type that is different from the first metal type into asecond solvent; and adding a fixing agent to the second solvent,stirring the second solvent, and then separating out the supportmaterials from the second solvent; and calcining the support materialsseparated out from the second solvent to produce the supported catalyst,wherein at least one of the first and second fixing agents comprise amonomer.
 15. The method according to claim 14, wherein the monomer is ofa type that can be polymerized in the solvent to form oligomers orpolymers, or both.
 16. The method according to claim 14, wherein thefixing agent added to the first solvent comprises acrylic acid and themetal salt comprises platinum salt.
 17. The method according to claim16, wherein the fixing agent added to the second solvent comprisesascorbic acid and the metal salt comprises palladium salt.
 18. Themethod according to claim 14, wherein the fixing agent added to thefirst solvent comprises ascorbic acid and the metal salt comprisespalladium salt.
 19. The method according to claim 18, wherein the fixingagent added to the second solvent comprises acrylic acid and the metalsalt comprises platinum salt.
 20. The method according to claim 14,wherein the support materials are made of lanthanum-doped alumina.